The doses of folic acid, necessary to avoid folic acid deficiency in patients being maintained on hemodialysis, have been estimated to be between 1 and 5 mg daily. To more precisely define an adequate dose of folic acid, 6 anephric patients were studied. The patients were maintained on hemodialysis and received, in a crossover fashion, 1 mg and 5 mg of folic acid for 3 wk. In a second crossover study, 3 anephric patients were first maintained on 0 and then on 1 mg of folic acid after each dialysis for 3 wk periods. Pre- and postdialysis folic acid blood levels were measured and dialyzer clearances of folic acid were determined. The results of this study support the conclusion of the Food and Drug Administration report suggesting that daily doses of 1 mg of folic acid are adequate to sustain therapeutic folate levels. The data further indicate that the administration of 1 mg of folic acid after each dialysis, rather than 1 mg of folic acid daily, can provide adequate folate.